Hello, I also got Animal Crossing: Wild World but I haven't played it in a while. But for your question, I found this guide on gamefaqs that might help you. Here is a small excerpt that fits the exact problem you had:
"Trees are perhaps the most tricky part of perfecting your town. It can get
quite frustrating when you go to the post office and are told that there are
not enough trees, so you plant one tree, go to the post office again, and
you're told that there are too many trees. That happens because of spacing.
Spacing is key when planting trees in your town. Sorry, but this may mean that
you have to get rid of your beloved "orchard." Again, there are rumors going
around that you should divide your town into 16 x 16 "acres", but there are
easier ways of doing it. First of all, make sure that no two trees are
touching each other. The rule is, if you can walk between two trees, they are
not touching. If you have trees in your town that are touching, chop one down
and dig up the stump. Pretty simple so far. Next, you need to make sure that
the trees are dispersed thoroughly throughout your town. Too many or too few
trees in a certain area will result in a non-perfect rating."
Basically its telling you to make sure no two trees are touching each other and trees should be evenly distributed through the town.
In the source, theres the link of the guide that could help you with your tree problem and maybe other future problem you might run into for making a perfect town.
Good luck!
